After skimming the moon’s surface for nearly a year, NASA’s two Gravity Recovery and Interior Laboratory (GRAIL) drones ended their mission on Monday by crashing into a mountain located at the Moon’s north pole. While it might sound like an accident, the crash was actually a planned maneuver.

The decision to crash the two spacecraft, Ebb and Flow, was made after NASA's scientists evaluated that the drones could no longer function with low fuel levels. More importantly, if the probes were to be left in low orbit, there was the possibility that they could damage one of the previous Apollo or Soviet lunar lander sites.

The impact, however, marks a successful end to the GRAIL project to survey the Moon’s surface with target sites pointed out by middle-school students. Over the last year, the two drones have captured over 115,000 photos of the lunar surface using a MoonKAM camera.

Not only did the satellites produce a new library of images, they also observed how the topography changed the Moon's gravity field whenever they passed over features such as mountains and craters.

Up to their last dying engine burns, the probes also helped engineers validate fuel consumption computer models that will help them plan out future space missions. NASA has named the site where Ebb and Flow impacted the moon in honor of the late astronaut Sally K. Ride, America's first woman in space as well as the leading proponent of the MoonKAM program.